The present invention relates to a single-chip microcomputer, and particularly to preservation of security of information for such a microcomputer.
A single-chip microcomputer comprises a CPU (central processing unit) comprising a control section, an ALU (arithmetic logic unit) and registers; a storage unit comprising a ROM (read-only memory) and a RAM (a read-write memory), and an I/O device (input/output device), which are all incorporated in a single chip of as an integrated circuit.
In a microcomputer used in an IC card (microchip card), an EEPROM (electrically erasable and programmable ROM) is used as part of a storage unit and is used for storing data, such as personal information, that should be kept secret. The contents of the EEPROM should not be divulged to outside parties. But during testing in the process of manufacture of the IC card, the EEPROM must permit reading of its contents. Accordingly, the capability of reading the contents of the EEPROM is essential at least until the tests are completed. To satisfy these contradictory conditions, a special mode, called the test mode, is provided in which tests on the EEPROM and other components are performed, and when the tests are completed, the microcomputer is changed into a state where the tests can no longer be performed. This is achieved for example by blowing a fuse forming part of the electrical circuit of the microcomputer.
The above arrangement is disadvantageous in that blowing a fuse requires a high energy and a special process. Moreover, once the microcomputer has been changed by blowing a fuse to a state where the tests cannot be performed, it cannot be returned into the original state, so that further tests or program analysis cannot be performed.